As a typical apparatus for measuring the total luminous flux emitted from a light source, a photometer including an integrating sphere is known. The integrating sphere has on its inner wall a reflecting surface formed by application of a diffuse reflecting material (such as barium sulfate or PTFE (polytetrafluoroethylene) for example). A light source to be measured (hereinafter also referred to as “sample light source”) is lit in the state of being arranged in this integrating sphere. The luminous flux emitted from the light source is repeatedly reflected from the reflecting surface of the integrating sphere's inner wall and accordingly the illuminance on the integrating sphere's inner wall is made uniform. The fact that the illuminance which has thus been made uniform is proportional to the total luminous flux emitted from the light source is utilized to measure the total luminous flux of the light source. Since the total luminous flux measured by such a photometer is commonly a relative value, the value of the total luminous flux is compared with a detected value (standard value) obtained by using a known standard light source to thereby measure a calibrated value of the total luminous flux emitted from the sample light source.
The photometer including such an integrating sphere cannot avoid absorption of light by parts such as a support member used for arranging the light source in the integrating sphere and a light shielding plate (baffle) used for preventing the light from the light source from directly irradiating a photodetector. Further, the sample light source itself absorbs light.
In order to address such light absorption, Japanese Industrial Standards JIS C 8152: 2007, “Measuring Methods of White Light Emitting Diode (LED) for General Lighting,” Jul. 20, 2007 (NPL 1) discloses that a factor for correcting for self absorption of a sample light source (self absorption correction factor) is used. This self absorption correction factor is calculated in the following manner. A light source for measuring self absorption (typically incandescent or tungsten halogen lamp) is provided in an integrating sphere. Under respective conditions that the sample light source is arranged in the integrating sphere and the sample light source is not arranged in the integrating sphere, the light source for measuring self absorption is lit. The values detected under respective conditions are compared with each other to thereby calculate the self absorption correction factor.
Further, with the purpose of avoiding influences of light absorption by the support member for example, a photometer including a hemispherical integrator (hereinafter also referred to as “integrating hemisphere”) like the one disclosed in Japanese Patent Laying-Open No. 06-167388 (PTL 1) has been proposed (the photometer will also be referred to as “hemisphere photometer” hereinafter). This hemisphere photometer has, instead of the integrating sphere, an integrating hemisphere made up of a hemispherical portion having a reflecting surface formed on its inner wall and a circular mirror plate arranged so that its reflecting surface covers the opening of the hemispherical portion. A light source is disposed at the center of the mirror plate so that the center of the light source coincides with the center of curvature of the hemispherical portion.
In such a configuration, the light source and a virtual image of the light source generated by the minor plate are each present in an imaginary integrating sphere (a composite space made up of the real space in the hemispherical portion and the virtual image of the hemispherical portion). Namely, the hemisphere photometer enables the sample light source to be arranged in the imaginary integrating sphere without using the support member for supporting the light source. An error due to absorption of light by the support member for example can thus be reduced.